With the large growth in the use of wireless devices, in particular those devices which operate in the unlicensed frequency bands, a demand is being created for radios which are able to identify and cope with other, potentially hostile, signals which also operate in those bands. The goal is not only to allow a particular radio to communicate successfully in the presence of interference, but also to minimize the effect of radio's own transmissions has on other systems which are operating in the same band.
This is a very difficult goal to achieve when one considers that in unlicensed bands radios can transmit intermittently, and in the case of frequency hopping radios, also exhibit dynamic behavior by constantly changing their frequency of operation over time. Throw mobility into the mix, and what you have are widely varying power levels for various interferers seen by a particular radio operating at a constant point in space over time. Thus the licensed radio paradigm, where a system sets up a wireless network at some known frequency and stays on that frequency for the duration of its operation, will just not work in unlicensed radio bands.
Various schemes exist for coping with varying interference levels, most notably Frequency Hopping Spread Spectrum (FHSS), which is widely in use. In FHSS systems, the frequency of operation is changed regularly according to some random pattern. FHSS was originally designed to be an anti jamming mechanism—it does mitigate the effects of interfering signals on average over time—the random (essentially blind) nature of the choice of centre frequency, and the regularity of the hopping does limit its performance. While over time, the effects of interference are mitigated, during any single hop the link may not function. The duration of a single hop can be in the hundreds of milliseconds, which, for a high-speed communications system, is a large number of bits. During a single hop, the radio link could be inoperable depending on what the interference is doing at the time. Whether or not a particular hop frequency will permit the system to operate is also largely unknown, since interferers can be intermittent and change their centre frequency. This leads to radio “dead spots” with FHSS, where no data is being received while the system is waiting to hop to a different frequency.
Examples of systems exist in the prior art which use frequency information to optimize their transmission. One method which is particularly common to systems which regularly change their centre frequency (called Frequency Hopping Spread Spectrum, or FHSS) is to test each channel's integrity by sending data on the channel's centre frequency. If relatively error-free data is received, then that channel is deemed as a “good” channel for transmission. If many errors are seen, or data cannot get through at all on that channel, then that channel is flagged as a “bad” channel. A table is kept of channels which are “good” and “bad”, so that only the ones which have been proven “good” may be used.
This method has inherent drawbacks, however, since it not only relies on a radio which changes its centre frequency often and regularly (and is thus suitable only for FHSS systems), but it also uses the actual data transmission as the benchmark for channel quality. For this system to work, data must be periodically transmitted over “bad” channels where communications will be degraded or interrupted. Channels must also be regularly re-tested, which can contribute to communications interruption. Such a system is also unable to determine what, exactly is causing the data interruption, so that degradations caused by interference cannot be differentiated from channel effects, system delays, power interruptions, etc.
Another method which can be used by fixed-frequency systems is a technique called collision avoidance, where the system listens for other signals on the channel it is using, and if it detects a large amount of traffic, uses another channel. In such a scheme, only radios using the same protocol as the radio testing the channel can be detected. Other interferers and physical effects of the channel cannot be detected with this method.
Another passive method which can be used to detect interfering signals is to use a Received Signal Strength Indicator (RSSI) which already exists on the radio for Automatic Gain Control functions. The radio waits for a dead period in between transmissions and uses the RSSI to measure the power coming in from the channel. If that power is above a certain threshold, the channel is deemed as having too much interference and another channel is used. While this method is simple and fast, it provides no information about the nature of the power being detected.
It is possible to produce a frequency scan of the band of operation without having to “test” the channel, using Fourier Transform techniques. Creating a Fourier Transform of the entire band gives detailed information about not only the signals in the band, but also the behavior of the radio channel. Unfortunately quite a bit of extra hardware has to be added to a conventional communications system to achieve this, which increases the complexity, power consumption, and cost. In effect, you would be adding the capability of a spectrum analyzer to the radio, which in many applications (particularly low power ones) is impractical.
What is needed for a radio to function reliably in an unlicensed band is a scheme which allows it to measure the properties of all the interfering signals on the band quickly and accurately and use this information to make intelligent decisions about its own frequency of operation, transmission time periods, and other factors. These decisions could allow the radio to transmit more reliably than if it “blindly” chose its centre frequency, or waited for the loss of its own data link to do something about the interference.